Doublelayered facade elements

Double layered concrete facade panels can be made with or without a cavity and with a load bearing or non load bearing inner leaf.

The cheapest method is without a cavity. During manufacturing the cast outer layer can stay in its mould, face down, but this mould is already adapted for the double construction. Stainless steel anchors

I. I I Device for controlling cover to reinforcement in face-down casting.

project from the outer layer Thermal insulation material is put on the back of the outer layer This now acts as the bottom of the mould for the inner leaf. Reinforcement and anchors can be put in as in single layer construction.The inner leaf is then cast on top of the thermal insulation.The inner side of the element will stay somewhat rough.

If a cavity is needed the process is more complex. First the inner leaf is made and demoulded. Stainless steel anchors are cast in and project outwards.Thermal insulation is attached to the inner side of this inner layer Then the outer layer is cast in another mould. After that, the inner leaf, with the projecting anchors, is pushed into the not yet hardened concrete. Spacers between the inner leaf and the mould of the outer blade maintain the desired distance between the outer layer and the thermal insulation. Although the latter solution is more expensive it has some advantages,for instance both faces of the facade element have a mould side with a high degree of finish.The outer side needs no treatment at all; the inner side is often painted.The other advantage is the cavity, especially when a damp-proof outside finish is used, like (glazed) tiles.The risk of damage to the tiles by frozen condensate accumulating behind the tiles is reduced by the ventilation in the cavity. In most cases, a cavity is not needed.

The risk of fire propagation in the cavity can be reduced by using mineral wool. Foams like polystyrene and polyurethane are however virtually waterproof and the concrete can be cast on to them directly. If a fire-retardant type of foam is used in a construction with a cavity, it reduces the risk of the fire spreading quickly through the cavity, as the supply of oxygen will be reduced. As the outer leaf hangs on steel anchors, the material, design and position of these anchors must be chosen taking a possible fire into account.The problems of different thermal movement between the outer and the inner leaf can be solved by using a fixing somewhere in the centre of the panel, and pendulum anchors in the form of steel plates on the four corners. These plates allow the panel to expand and shrink, but prevent rotation (Fig. 1.12).

Joints

The water sealing in the outer leaf can be made as described in single layer construction. Attention must be paid to the air seal. As both blades are placed at once it is not possible to make an air seal on the cavity-side of the inner leaf. Attaching a broad tape containing tar over the joint is the best and cheapest solution.The joint must be made by sealants or rubber profiles that maintain their flexibility over the years.

If the inner leaf is load bearing a conflict might arise between the accuracy in position of the element one can reach in the rough construction to position the element and the accuracy needed to reduce the width of the joints in the outer leaf to an acceptable level.

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1.12 Placement and type of anchors in double layered facade element. In the middle is a fixed anchor In the four corners are steel plates that can deform slightly perpendicular to their plane but prevent rotation

1.12 Placement and type of anchors in double layered facade element. In the middle is a fixed anchor In the four corners are steel plates that can deform slightly perpendicular to their plane but prevent rotation

Tolerances 19

Tilting moulds

Tilting moulds are ideally suited to casting panels of uniform profile. Not all coffered panels can be satisfactorily demoulded. Because handling stresses are less after the panel has been tilted to a vertical position, certain lifting provisions can be omitted and the required quantity of reinforcement for handling can be reduced: in which case the units must be handled vertically during transportation and erection. Thus panels made in tilting moulds are generally thinner than those produced using other casting methods. Care must be taken to ensure that there is adequate reinforcement to resist wind loading when fixed in place.

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